1. Field of the Invention
The present invention relates to a method and an apparatus for burning gaseous fuel, like coal gas that is formed by coal gasification, wherein the fuel composition varies with the kind of fuel source used. Especially, the invention relates to a combustion method and apparatus by which stable combustion can be maintained even when the composition of gaseous fuel varies.
2. Description of the Prior Art
As described in Japanese Patent Application Laid-Open No. 57-172,229, a fuel nozzle system of the prior art comprises gaseous fuel passes and air passes arranged alternately adjacent one to another on a pitch circle, all the passes being provided with injection ports for causing the same-direction turning of fuel and air, and the gas injection ports having such areas that the dynamic pressure of gaseous fuel at the maximum flow may be equal or lower than the dynamic pressure of the air fed through the air passes.
In this system, the area of fuel injection ports is defined on the basis of the dynamic pressure of the air fed through the air passes. However, such systems are not designed by considering the case where variation occurs in the amount of air fed through the air nozzles or in the amount of inert gas present in the fuel gas. Possible variations in the conditions include the exchange of the area of air distributor ports in the burner and variation in the fuel composition. In particular, variation in the fuel composition is accompanied by variation in calorific value per a unit volume of fuel and hence the whole amount of air varies and the dynamic pressure of the air fed through the air passes. Under such conditions, variation occurs in the degree of fuel-air mixture or in the magnitude of circulating streams developed in the downstream of the fuel nozzle. These variations result in unstable flame.
In particular, gaseous fuel from a coal gasifier, that is, coal gas from a gas producer varies largely in gas composition and in calorific value with the species of raw material coal. Therefore, it is extremely difficult to obtain stable flame from coal gas by using one combustion apparatus.
In a gas turbine power plant wherein gaseous fuel from a gas producer is used, it is impossible to stop the gas turbine and exchange the fuel nozzle of the gas turbine burner or the burner itself, every time the species of raw material coal changes. In order to commercialize such a gas turbine power plant in the future, it is indispensable that the gas turbine burner can be operated continuously regardless of the species of coal gas to charge into the gas producer.
As to the prior art analogous to the present invention, there is known a powdered fuel injection burner. This burner, for instance, is provided with a nozzle system for charging powdered fuel into a gas producer and a plurality of ports for injecting a gasifying agent (e.g. air). These ports are designed so that the number of open or closed ports thereof may be controllable for the purpose of keeping the speed of injecting the gasifying agent nearly constant, this flow speed being the main factor having great effect on the gasification efficiency, even when the ratio of the powdered fuel and the gasifying agent is changed according to variation in load on the gas producer. That is, the burner has such a mechanism that the flow rate of the oxidizing agent to feed. Generally, gas turbine burners are operated with the air (oxidizing agent) flow being kept nearly constant regardless of the load. Especially in gas turbine power plants wherein coal gas is used as fuel, loads on turbines employing coal gas as fuel are at least 30% (less loads than 30% pose a problem in the stability of combustion) in most cases and the turbines will be operated at nearly constant flows of air up to 100% load. In addition, the gas temperature under varying turbine loads will be regulated by fuel control alone while the control of the amount of air or the control of the flow rate of injected air will not relate directly to turbine loads. Accordingly, the control of fuel flow will be important in gas turbine power plants.
Among gas turbine burners burning common fuels, e.g. natural gas, there is an example wherein fuel is charged in two stages to reduce the concentration of NOx discharged. Because of the high combustion rate, a good quality fuel such as natural gas can be burnt up in a short time even when charged into a mid zone of the burner. In contrast to this, a fuel such as coal gas exhibiting a low rate of combustion needs to be burnt up by maximizing the gas fuel residence time in the burner. Accordingly, it is most ideal to charge such a fuel at the top (up-stream side) of the burner.
The above stated prior art does not take into consideration the stability of flame to be maintained when the composition of fuel varies; hence there are problems in applying the above prior art to actual gas turbine power plants.